Summary1. In coastal and estuarine systems, foundation species like seagrasses, mangroves, saltmarshes or corals provide important ecosystem services. Seagrasses are globally declining and their reintroduction has been shown to restore ecosystem functions. However, seagrass restoration is often challenging, given the dynamic and stressful environment that seagrasses often grow in. 2. From our world-wide meta-analysis of seagrass restoration trials (1786 trials), we describe general features and best practice for seagrass restoration. We confirm that removal of threats is important prior to replanting. Reduced water quality (mainly eutrophication), and construction activities led to poorer restoration success than, for instance, dredging, local direct impact and natural causes. Proximity to and recovery of donor beds were positively corre- The meta-analysis shows that both trial survival and seagrass population growth rate in trials that survived are positively affected by the number of plants or seeds initially transplanted. This relationship between restoration scale and restoration success was not related to trial characteristics of the initial restoration. The majority of the seagrass restoration trials have been very small, which may explain the low overall trial survival rate (i.e. estimated 37%). 4. Successful regrowth of the foundation seagrass species appears to require crossing a minimum threshold of reintroduced individuals. Our study provides the first global field evidence for the requirement of a critical mass for recovery, which may also hold for other foundation species showing strong positive feedback to a dynamic environment. 5. Synthesis and applications. For effective restoration of seagrass foundation species in its typically dynamic, stressful environment, introduction of large numbers is seen to be beneficial and probably serves two purposes. First, a large-scale planting increases trial survival -large numbers ensure the spread of risks, which is needed to overcome high natural variability. Secondly, a large-scale trial increases population growth rate by enhancing selfsustaining feedback, which is generally found in foundation species in stressful environments such as seagrass beds. Thus, by careful site selection and applying appropriate techniques, spreading of risks and enhancing self-sustaining feedback in concert increase success of seagrass restoration.

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Knowledge of the natural pattern of spatial variation of the variables of interest is fundamental for designing effective sampling strategies to be used in surveys and monitoring programs. Unfortunately, little is known on patterns of spatial variation of structural, morphological and dynamic features of Posidonia oceanica, despite their increasing use as indicators of the status and/or trend of meadows. Here, variations in some such features (such as shoot density, leaf biometry, annual rhizome elongation and leaf formation) were examined on a hierarchy of spatial scales, ranging from cm within individual meadows, to meadows separated by 10 km, using independent full-nested analyses of variance. Overall, at least 3 variables showed significant variations in the mean values on each of the scales examined. Patterns of variability differed among individual traits. Most of the total variation, however, was on the smallest spatial scale examined (i.e. among replicates, shoots or quadrats). For some variables, a significant variation was also associated with the largest scale (i.e. among meadows). Collectively, the results indicated that patchiness in density, morphology and growth exists on a very local scale within P. oceanica meadows along the same isobath. This suggests that sampling designs and analyses which incorporate different spatial scales and appropriate replications should at least be considered in the future when planning large-scale and long-term surveys.

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